This invention relates to tension gauges, and especially to a tension gauge for indicating proper tension on a fastener such as a mine roof bolt. The tension gauge according to the invention indicates not only overtension on the bolt, but undertension and proper tension as well.
It is well known in the mining art that it is necessary to reinforce formations of rock and other strata above the mine roof. It is conventional to drill bore holes in the mine roof and insert therein mine roof bolts which are on the order of six or more feet in length. The bolts are anchored on one end by expansion means and are provided with a plate which is pressed against the mine roof by torquing the mine bolt into the hole. As the bolt is rotated, it causes the expansion means to secure the bolt in the hole. As the bolt is tightened by rotation, it is subjected to tension. However, if the bolt is not properly torqued, undertension results and if the bolt is over-torqued, overtension results. In either case, such an installation is unsatisfactory for it means that an unsafe condition is created wherein the mine roof could fall on personnel working in the mine.
Presently, it is the usual practice to hand check tension on mine roof bolts with a torque wrench. However, hand checking involves a potential safety hazard to the checker since he is unprotected; it is also cumbersome and time-consuming. Therefore, efforts have been made to provide some sort of simple means for indicating when a mine roof bolt is properly tensioned. Heretofore, tensioning devices have been unsatisfactory because they indicate only when a bolt is overtensioned, that is, when it is torqued too much. Other tensioning gauges have been unduly complicated and therefore are expensive to manufacture.
It is, of course, desirable that such gauges be very inexpensive since thousands and thousands of them are used, that they not be unduly complicated, and that they be disposable. They should be easy for the operator to see and must be designed to deform at the proper tension.
Prior art tensioning devices include those disclosed in the following U.S. Pat. Nos. 2,824,481; 3,329,058; 4,137,816; 3,104,645; 4,170,163; 3,174,386; 3,161,174; 4,047,463; and 2,943,528. A typical tension or stress indicating device is disclosed in U.S. Pat. No. 3,174,386, which comprises a washer means which is disposed coaxially adjacent a roof bolt between a mine roof plate and a driving head or washer associated therewith. The washer means consists of an axial and radial expanse of solid compressively-plastic material, such as mild steel or high strength nonferrous metals. According to the patent, the material has a plastic yield strength less than that of any of the other parts of the fastener with the expanse being adapted to plastically deform axially and radially under fastener tension and take a permanent plastic set so as to directly respond to and sense the tension to permit the operator to ascertain the tension. The preferred washer constitutes a spacer and sensing member which is a simple hollow right cylinder having an axial extent greater than that necessary to space the adjacent pressure plate from the adjacent one of the connected surfaces, namely the roof bolt head, when the roof bolt is tightened, the excess extent of the cylinder of the washer having a direct precalibrated proportion to the predetermined installation tension of the roof bolt when the cylinder is axially compressed by the roof bolt head sufficiently to eradicate the excess extent. The axial shortening of the cylinder sufficient to eradicate the excess length exceeds the limiting elastic strain of the cylinder and effects indication of the predetermined tension. In the patent, it is disclosed that the right cylinder includes a central portion which is reduced relative to the outside axial portions such that upon tightening of the roof bolt, a crimp is formed which provides a visual indication to the operator that the bolt is under tension. However, it is not possible to state whether when the crimp is fully collapsed the bolt is properly tensioned and it is not possible to tell from visual observation whether the bolt is undertensioned because it is not possible to determine the extent of the crimp. In any event, although the patentee states that when the crimp has been flattened, rotation of the bolt should cease as further rotation will then increase the tension in the bolt beyond the ultimate yielding capacity of the device, as a matter of fact, when the crimp is formed, tension on the bolt is released and there is nothing to indicate that proper tension will be obtained again.